1. Field of the Invention
The present invention relates to a sheet storing apparatus, and more particularly to a sheet storing apparatus for stacking sheets ejected from an image forming apparatus such as an electrophotographic copying machine or a laser printer on a tray.
2. Description of Related Art
In a laser printer, a large number of sheets are used in a printing operation, and therefore various sheet storing apparatuses having a capacious stacking table such as a tray or a stacker have been provided for laser printers. Generally, such a sheet storing apparatus is so made that the stacking table moves downward intermittently as a sheet stack thereon is growing, and therefore the sheet storing apparatus is capacious. Incidentally, sheets are different in weight, depending on the size, and a stack of a larger size of sheets is heavier than that of a smaller size of sheets when they are the same height. A limit is set to the height of a sheet stack to be placed on a stacking table, and generally the height limit is determined depending on how high the largest size of sheets can be stacked on the stacking table without damaging the stacking table, its supporting means and driving means. In stacking a smaller size of sheets on the thus designed stacking table, the stack comes to the height limit although the stacking table is strong enough to receive more sheets.
U.S. Pat. No. 4,927,131 discloses that a limited number of sheets can be stacked on a stacking table and that the limit is varied in accordance with the size of sheets. According to U.S. Pat. No. 4,927,131, different limits are predetermined from size to size, and sheets stacked on the stacking table are counted. However, this system does not comply with a case where some sheets are taken out of the stack during a counting operation, and the count value becomes different from the actual number of sheets on the stacking table.
The reason why the stacking table is moved down with growth of a sheet stack thereon is to keep a certain distance between a nipping portion of rollers for ejecting sheets from the image forming apparatus onto the stacking table and the top surface of the sheet stack on the stacking table. Thereby the sheets on the stacking table can be maintained in alignment. The device disclosed in U.S. Pat. No. 4,927,131 has a sensor for detecting a sheet on a tray and a sensor for detecting the top surface of a sheet stack on the tray. In the device, it is judged from the outcome of the top surface sensor that some sheets are taken out of the tray, and in this case the tray is moved up.
However, in this type of device, the tray is not always moved up in response to a decrease of a sheet stack on the tray. When an operator lifts up the leading ends (in the sheet storing direction) of the sheets so as to discharge the sheets from the tray, the sheet sensor detects no sheets on the tray, while the top surface sensor detects the sheets lifted by the operator and judges that there is no space for more sheets on the tray. In this case, the tray is not moved up when the operator finishes discharging the sheets from the tray.
In this type of device, when the top surface sensor detects the top surface of a sheet stack on the tray, the tray is moved down immediately. By this downward movement, the distance between the nipping portion of the ejection rollers and the top surface of the sheet stack on the tray is so elongated that the next transported sheet may not be aligned on the tray. Especially in a device wherein a paddle wheel is disposed coaxially with the ejection rollers so as to align a sheet being transported from the ejection rollers onto the tray, immediately after the tray is moved down, the blades of the paddle wheel cannot reach the sheet transported onto the tray, and the sheet cannot be aligned.